Wireless systems typically include a transmitter and receiver coupled to an antenna to send and receive Radio Frequency (RF) signals. Generally, a baseband system generates a digital signal that includes encoded information (data), and the digital signal is converted to an analog signal for transmission. The analog signal is processed and typically modulated (up converted) to an RF carrier frequency. After up conversion, the RF signal is coupled to an antenna through a power amplifier. The power amplifier increases the signal power so that the RF signal can communicate with a remote system, such as a base station, for example.
Wireless systems generally use power amplifiers that can transmit RF signals with a considerable range of instantaneous and average signal power levels. Generally, power amplifiers are sized and designed such that the optimal efficiency is only at the maximum instantaneous output power, resulting in a significant reduction in efficiency for dynamic signals. As such, advanced architectures that provide higher efficiency at output power levels other than the maximum instantaneous output power are sought.
One example amplifier architecture that finds use in RF systems is a Doherty amplifier. A Doherty amplifier includes a main power amplifier stage, usually a class A or AB power amplifier, and a peak power amplifier stage, usually a class C power amplifier. It would be desirable to increase efficiency of the Doherty amplifier in order to maintain performance by using less power, especially over a variety of power supply levels.